1. Field of the Invention
The present invention relates to a mechanism for suppressing the displacement or vibrations during travelling of a travelling crane such as a rough terrain crane or the like.
2. Description of the Prior Art
Travelling cranes are generally constructed as shown in FIG. 7, in which on a car body 2 which is supported by wheels 1 there is supported a boom 3 rotatably around a horizontal shaft 5 through a boom raising and lowering hydraulic cylinder 4. In such a travelling crane, when the car body 2 vibrates during travelling due to ruggedness of the road surface or due to sudden acceleration or deceleration, the boom 3 rocks vertically, so that the vibration of the car body 2 is enhanced and riding on the car body becomes less comfortable.
As an apparatus for suppressing such vibration during travelling there is known, for example, the apparatus disclosed in Japanese Patent Laid-Open No. 18295/1984. As shown in FIG. 8, this known apparatus is provided with a damping mechanism 19 in the interior of a boom raising and lowering hydraulic cylinder 18. Also provided are a counterbalance valve 12 in an oil path 13 connected to an oil chamber 181 which holds the load of the cylinder 18, as well as an electromagnetic change-over valve 16 and a shuttle valve 15 both disposed between the oil path 13 and an oil path 14 connected to both an oil path 17 and a direction control valve 11, the oil path 17 being connected to another oil chamber 182.
According to such known apparatus, during operation of the crane, when the direction control valve 11 is changed over to a boom UP position or DOWN position while the change-over valve is in "A" position, discharged oil from a pump 10 flows into the oil chamber 181 or 182 of the cylinder 18 to cause the cylinder 18 to be extended or withdrawn, whereby the boom is brought up or down. In travelling of the crane, the cylinder 18 is extended or retracted while the change-over valve 16 is held in "A" position to set the height from the ground up to the front end of the boom to an appropriate travelling height, and thereafter the change-over valve 16 is switched to "B" position, whereby the oil path 13 is brought into communication with the oil path 17 through both the change-over valve 16 and the shuttle valve 15, and the oil chambers 181, 182 and the damping mechanism 19 are communicated with each other to form a closed loop. Consequently, the load pressure acting on the oil chamber 181 of the cylinder 18 is conducted to an oil chamber 191 of the damping mechanism 19 and accumulated therein to exert a displacement suppressing action during travelling.
In travelling cranes of this type, in order that the displacement suppressing action may be exhibited effectively, it is necessary to set the height of the boom 3 a little higher than a bottom dead center H.sub.0 and a litter lower than a height H.sub.3 specified in the law in connection with travelling on general roads. More particularly, an appropriate height range of the boom 3 in travelling of the crane is from H.sub.1 to H.sub.2, provided H.sub.1 &gt;H.sub.0 and H.sub.2 &lt;H.sub.3. And it is necessary to maintain the boom height within that range.
In such conventional apparatus, however, even if the height of the boom 3 is set within the above appropriate travelling range of H.sub.1 to H.sub.2 in the initial state of travelling of the crane, subsequent drop of the oil temperature in the closed loop including the cylinder 18, that is, contraction of the oil, or unavoidable oil leakage with the lapse of the travelling time causes withdrawal of the cylinder 18, resulting in that the boom height becomes lower than the lower limit H.sub.1 of the above appropriate travelling range and finally reaches the bottom dead center H.sub.0, that is, the displacement suppressing action can no longer be exhibited. To avoid this, it is necessary for the operator to always check whether the height of the boom is within the appropriate travelling range of H.sub.1 to H.sub.2 or not. Thus, the operator cannot devote himself to the travelling operation, so his fatigue increases. Further, every time the height of the boom 3 becomes lower than the appropriate travelling range, it is necessary to change over the direction control valve 11 to extend or retract the cylinder 18, thereby raising or lowering the boom 3 for the adjustment of its height. But this operation is very troublesome. Besides, since the said adjustment of the height is not fully accurate, it is likely that the height of the boom 3 will exceed its limit height H.sub.3.